A type of mass spectrometers commonly used is the atmospheric pressure ionization mass spectrometer, which ionizes a sample under a gas pressure equal or approximate to the atmospheric pressure. Examples of this type include the electrospray ionization mass spectrometer (ESI-MS), the atmospheric chemical ionization mass spectrometer (APCI-MS), the atmospheric pressure matrix assisted laser desorption/ionization mass spectrometer (AP-MALDI-MS), the inductively coupled plasma mass spectrometer (ICP-MS) and the ion mobility spectrometry mass spectrometer (IMS-MS).
For example, in an electrospray ionization mass spectrometer, a liquid sample to be analyzed is sprayed from an electrospray nozzle into an ionization chamber maintained at or close to atmospheric pressure. The molecules of the sample turn into ions in the course of the evaporation of the solvent contained in the sprayed droplets. The ions thus produced are transported through one or more intermediate vacuum chambers into an analyzing chamber whose interior is maintained in a high-vacuum state. The analyzing chamber encloses, for example, a quadruple mass filter or a similar mass analyzer for separating the ions with respect to their mass to charge ratios. A detector then detects some of the ions thus separated.
The mass spectrometer having such a construction includes an ion lens, also called the ion optic, which accelerates and focuses energetic ions by means of electric fields. There are various types of ion lenses having different forms and constructions.
For example, the mass spectrometer disclosed in the U.S. Pat. No. 4,963,736 uses an ion lens composed of four pieces of rod electrodes to which only a radiofrequency (RF) voltage is applied. Another example is the mass spectrometer disclosed in the U.S. Pat. No. 6,744,047, which has six rod electrodes positioned around the ion beam axis and an RF voltage, superimposed on a DC voltage, is applied to the rod electrodes.
These types of ion lenses using rod electrodes are capable of focusing ions traveling through the space surrounded by the rod electrodes but not accelerating the ions along the ion beam axis. Therefore, if the ion lens is located in a low-vacuum atmosphere, or under a relatively high gas pressure, the ions can lose a significant proportion of their kinetic energy due to collisions with residual gas molecules. Some ions may even lose all their axial velocity before they have been transmitted through the ion optic. As a result, it is difficult to improve the ion transport efficiency of the ion lens.
In contrast, the mass spectrometer disclosed in the U.S. Pat. No. 6,462,338 uses an ion lens composed of multiple virtual rod electrodes positioned around the ion beam axis, where each of the virtual rod electrodes is composed of a plurality of separate metallic plate electrodes aligned in a row along the ion beam axis. Each of the plate electrodes constituting a single virtual rod electrode is fed with the same high frequency AC voltage superimposed on a different DC voltage. The DC voltage creates a DC electric field having a potential gradient along the ion beam axis so that ions are accelerated by the DC electric field. Thus, the mass spectrometer is capable of not only focusing the ions by means of the RF electric field but also accelerating the ions along the axis of the ion optic by means of the DC electric field, so that the ion transport efficiency is improved.
The behavior of an ion traveling through the electric field created by the ion lens depends on the mass to charge ratio of the ion. In general, an ion having a large mass to charge ratio is less affected by the electric field than an ion having a small mass to charge ratio. Therefore, for an ion having a large mass to charge ratio to be focused and transported with a high level of efficiency, it is necessary to create an axially accelerating electric field having a large potential drop. Taking this into account, the above-described mass spectrometer is constructed so that the RF voltage has a smaller peak to peak amplitude and the DC voltage is set lower for an ion having a smaller mass to charge ratio, whereas the amplitude of the RF voltage is set larger and the DC voltage is set higher for an ion having a larger mass to charge ratio.
However, under conditions where the vacuum is as low as that in the first intermediate vacuum chamber of an atmospheric pressure ionization mass spectrometer, an excessive increase in the amplitude of the high frequency AC voltage or in the DC voltage is liable to cause an electric discharge between adjacent electrodes. This means that the amplitude of the RF voltage and the DC voltage, respectively, have upper limits. The presence of such limits prevents the provision of appropriate conditions for an ion having a large mass to charge ratio to be efficiently focused and transported. As a result, the efficiency of transporting an ion through the ion optic and introducing said ion into the mass analyzer is lower for an ion having a large mass to charge ratio than for an ion having a small mass to charge ratio. This is one of the factors that lead to a reduction in the sensitivity of the analysis.
In recent years, mass spectrometers have widened their application areas to cover the research, development and quality control in the fields of biochemistry or production of medicinal supplies. Particularly, atmospheric pressure ionization mass spectrometers are becoming increasingly popular in the aforementioned fields because of the inherent advantages of the so-called soft ionization. Samples to be analyzed in the aforementioned fields typically consist of proteins, peptides or other substances that have large molar weights. Also, it is often the case that the sample contains only a trace of the component to be analyzed, so that the mass spectrometer needs to have a high level of sensitivity. However, as explained earlier, none of the conventional mass spectrometers have adequate sensitivity to an ion having a large mass to charge ratio. Therefore, a new mass spectrometer capable of the aforementioned measurement is now strongly demanded.
In light of the above-described situation, the present invention intends to provide a mass spectrometer constructed so that the transport efficiency for an ion having a large mass to charge ratio is improved and the sensitivity of the analysis is accordingly enhanced while maintaining the voltage (or amplitude of the voltage) applied to the ion lens at levels which preclude electrical breakdown.